5.1.8 Glutamate Dehydrogenase (GLDH or GDH)
This is essentially a mitochondrial enzyme occurring in liver, kidney and muscle tissues
and the highest concentrations are present in the liver. The enzyme catalyses the
conversion of glutamate to 2-oxoglutarate, and plasma levels are generally low in most
species.
5.1.9 Gamma Glutamyl Transferase (GGT)
(Also called gamma glutamyl transpeptidase.) This enzyme catalyses the transfer of
gamma glutamyl groups from glutamyl peptides to an acceptor peptide or L-amino acid.
The highest concentrations of GGT are found in the kidney, pancreas and then liver. In
the rat kidney the level of GGT is approximately 200 times higher than the level found in
hepatic tissue. The enzyme is located in the brush border cells of the renal proximal
convoluted tubules and on the canicular surfaces of the hepatic parenchymal cells.
Despite its relatively high tissue concentrations in the kidney, plasma GGT does not
appear to alter following renal injury but urinary GGT measurements are helpful in
monitoring renal tubular damage.
In hepatic studies, plasma GGT can be used as an indicator of cholestasis even in rats
where plasma GGT levels are normally very low, often less than 2 IU/L. The use of GGT
as a marker of enzyme induction and of the presence of hepatic tumours is less predictive
in laboratory animals compared with data from human studies (Braun et al., 1987; Batt et
al., 1992; see Chapter 6).